Currently the Third Generation Partnership Project (3GPP) is evaluating the potential benefits of uplink transmit (Tx) diversity in the context of High-Speed Uplink Packet Access (HSUPA). With uplink transmit diversity user equipments (UEs) that are equipped with two or more transmit antennas are capable of utilizing all of them for uplink transmissions. This is achieved by multiplying a UE output signal s(t) with a set of complex pre-coding weights wi where i=1 . . . N with N denoting the number of transmit antennas. The rationale behind uplink transmit diversity is to adapt the pre-coding weights so that user and network performance is maximized. Depending on UE implementation the antenna weights may be associated with different constraints. Within 3GPP two classes of transmit diversity are considered:                Switched antenna transmit diversity, where the UE at any given time-instance transmits from one of its antennas only. Thus if wi≠0, wj=0 for all j≠i.        Beamforming where the UE at a given time-instance can transmit from more than one antenna simultaneously. By means of beamforming it is possible to shape an overall antenna beam in the direction of a target receiver.        
While switched antenna transmit diversity is possible for UE implementations with a single power amplifier (PA) the beam forming solutions may require one PA for each transmit antenna.
Switched antenna transmit diversity can be seen as a special case of beamforming where one of the antenna weights is 1 (i.e. switched on) and the antenna weight of any other antenna of the UE is 0 (i.e. switched off).
A fundamental idea behind uplink transmit diversity is to exploit variations in the effective channel to improve user and network performance. The term effective channel here incorporates effects of transmit antenna(s), transmit antenna weights, receiving antenna(s), as well as the wireless channel between transmitting and receiving antennas. Selection of appropriate antenna weights is crucial in order to be able to exploit the variations in the effective channel constructively. Furthermore timing of changes in antenna weights is important. When the antenna weights changes the effective channel as perceived by a receiving base station, such as a Node-B, may change abruptly. For instance, in a case of switched antenna transmit diversity with two antennas, if a UE starts transmitting on antenna 2 instead of antenna 1 then the corresponding antenna weights w1/w2 will change from 1/0 to 0/1 in an instant. As a result previous channel estimates become out of date, which may lead to worse effective signal-to-interference ratio (SIR) at the Node-B and packet errors until channel estimates have been updated and the Node-B has adjusted its channel estimates to reflect the new channel. Hence it is beneficial to avoid unnecessary changes of antenna weights or antenna switches.
It has been decided in 3GPP, as mentioned in 3GPP Tdoc RP-090987, 3GPP Work Item Description Uplink Tx Diversity for HSPA, to investigate uplink transmit diversity techniques for HSPA that do not require any newly standardised dynamic feedback signalling between network and UE. According to some suggested schemes the UE should monitor Transmission Power Control (TPC) commands received on the feedback channel F-DPCH (Fractional Dedicated Physical Channel) to select antenna weights. The F-DPCH is an already existing feedback channel and TPC commands is feedback information that is already transmitted for purposes of uplink transmit power control. For example, if a UE operating with switched antenna transmit diversity receives a large number of consecutive TPC UP commands on F-DPCH from the serving cell, the UE may conclude that it is likely to be beneficial to switch to another antenna for transmission.